Field of the Invention
The present invention relates to the field of measuring the geometry of multi-fiber optic ferrules and connectors by means of interferometric microscopes. More specifically, the invention relates to interferometric measurement of end face surface angle.
Description of the Related Art
Manufacturers of fiber optic connectors seek ways to produce the connectors with low transmission loss and low back reflection. End faces of fiber optic connectors must satisfy certain criteria for effective fiber mating as required by the industry standards. They must be clean and their surface geometry must provide for good physical contact and low signal loss.
Connector manufacturing procedure includes end face polishing for achieving the surface parameters that ensure good mating of the connectors in plugs. End face flatness needs to be verified that can be done by measuring the end face surface angle.
International Electrotechnical Commission standard IEC 61-300-3-30 defines surface angle as the angle between a reference plane which is perpendicular to the average guide pin axis and the connector end face surface.
The end face parameters, including the surface angle, are measured in an interferometric system. Special fixtures for positioning the connectors in the system are used. In case of multi-fiber connectors, the fixtures enable precise connector alignment in the interferometric system with the use of two guide pins. In the same way, in multi-fiber connector plugs two guide pins enable precise alignment between mating male and female connectors to minimize the optical insertion loss,
The fixtures are accurately adjusted and calibrated before measurement so that calibration angles of the pins relative to the optical axis of the interferometric microscope are known. The surface angle is calculated as the angle between the surface end face plane and a reference plane perpendicular to the optical axis of the interferometric microscope with known calibration angle correction.
Guide holes of multi-fiber connectors or ferrules are not strictly parallel. There is always some parallelism deviation as demonstrated on FIG. 1. On this figure the reference number 1 represents a multi-fiber connector or ferrule, the top image being the top view of the connector or ferrule and the bottom image being its side view.
Reference plane 5 is perpendicular to the optical axis 2 of the interferometric system. When guide pins 3 and 3′ of a fixture are inserted into the connector guide holes which have some deviation from being parallel, the guide pins become deviated too. The alignment angle of pins is changed. The reference number 6 demonstrates deviation angle of one of the guide pins. In the same way, the other guide pin has a deviation angle as well.
Since during the surface angle calculation the deviation angle is not taken into account, the measured angle value has some calculation error.
The present invention suggests measuring precise alignment angle of the guide pins by scanning them from side together with inspecting the connector end face in one measurement or in several subsequent measurement without re-inserting the connector or ferrule. The measured values of the angles between the guide pin axes are taken into account when calculating the surface angles. Such surface measurement method allows to increase measurement accuracy by considering the deviation angle of the guide pins in calculations.
There is a known method for determining precise orientation of the axis of guide pin holes of a multi-fiber ferrule and precise angle of the ferrule (see Dean et al. US Patent Publication US 2004/0101255)
The present invention employs the same technique of measuring fiber optic connectors as described in a related patent application by the same inventor (see Towfiq, F., (2015) U.S. patent application Ser. No. 14/697,784). The interferometric data is obtained from a side face of the guide pins which allows to measure the alignment angles of the guide pins of the connector or ferrule with two degrees of freedom.